Page 194 - Industrial Ventilation Design Guidebook

1 56 CHAPTER 4 PHYSICAL FUNDAMENTALS

solution are converted to insoluble oxides, which are removed by filtration.
The use of gaseous chlorine is not recommended if the water contains or-
ganics, or else car- cinogenic by-products can be formed (trihalomethanes).
Gaseous chlorine also presents health problems in the case of leaks and is
corrosive.

Filtration
If the solution is allowed to flow through a granular bed such as sand, the
larger particulate matter remains on the surface, while the smaller material is
collected in the thickness of the granular bed. Pressurization of the filter accel-
erates the process. Besides sand, other materials used as filtering media are an-
thracites, manganese dioxide, and activated carbon.

Softening
The sense of touch allows one to determine if water is hard or soft. For a
domestic application in a hard-water area, more soap is required to produce
lather than is required in a soft-water area.
The temporary hardness salts can be removed by boiling; these salts may
be classified as alkaline or carbonate hardness salts. These salts in solution
are calcium carbonate, CaCO 3, calcium hydrogen carbonate, Ca(HCO 3} 2,
and magnesium hydrogen carbonate, Mg{HCO 3 ) 2, Heating Ca(HCO 3 ) 2 pro-
duces water, carbon dioxide, and calcium carbonate, and this compound is
deposited on heat exchangers.
The salts that cause permanent hardness are calcium sulfate, CaSO 4, cal-
cium chloride, CaCl 2, magnesium sulfate, MgSO 4, and magnesium chloride,
MgCl 2. These are known as nonalkaline or noncarbonate hardness salts and
cannot be removed by boiling; they must be removed by chemical treatment.
The internal process complements the external process by taking care of
any contamination that may enter the water from the process.
The following is a brief introduction to the various types of water soften-
ing plants encountered.

Lime Soda. If carbon dioxide is in solution in water and calcium hy-
droxide is added, the resulting precipitation product is CaCO 3; this can be re-
moved by sedimentation.
If the water is temporarily hard due to the presence of Ca(HCO3) 2, and
calcium hydroxide is added, the resulting products will be a precipitate.
If the water is permanently hard due to MgSO 4, and lime is added, the
precipitates calcium sulfate, CaSO 4, and magnesium hydroxide, Mg(OH) 4, re-
sult, which are removed by sedimentation.
Permanent hardness can also be due to the presence of CaSO 4, in which
case the addition of soda (sodium carbonate), Na 2CO 3, produces sodium sul-
fate, Na 2SO 4, and calcium carbonate, CaCO 3; this precipitate once again is re-
moved by sedimentation.
Ion Exchange. When water flows through a resin ion exchange material
bed, some of the undesirable ions are adsorbed and replaced with less objec-
tionable ones. The process may be either